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Author Topic: Good stuff at the local HAM flea market !  (Read 747 times)

Posts: 29

« on: September 21, 2002, 05:06:08 PM »

Oh boy, this about cinches it. After today, I'm well on my way to legitimate HF progress.

Astron 35A supply, new, $100
Trap dipole 10,15,20,40, new $40
100' ladder line $2 (keep this for later)
50' RG8 coax $6

Here's the thing...

I'm a new Ham. I read somewhere to avoid long runs of low-eff. coax and also to avoid short runs of high-eff. coax. I assume its a bandwidth issue in regards to the short runs. I would like to make some 2-4' patch cords out of my RG8. Will this present any significant problems?

Thanks in advance...


Posts: 3331

« Reply #1 on: September 21, 2002, 06:46:45 PM »

I'm not sure what you mean by "eff." but I'll assume it means 'efficiency' ...

... what you probably read had something to do with low LOSS (which can be construed, I guess, as efficiency), and the reason you don't want long runs of lossy coax is so you don't lose your signals through it ... and the reason you can use thinner, higher-loss coax for short runs is because it is more physically flexible ... connecting a radio to a tuner, for example, with an RG-8u jumper my work, if both units are heavy, but if the tuner is light for instance, then the stiff cable may be more difficult to work with ...


Posts: 939

« Reply #2 on: September 21, 2002, 07:17:38 PM »

The short answer to your question is:
No, you won't have any problems using "RG-8 type" coax for short jumpers.  
(I use RG-213, etc. as jumpers all the time....I really like the mechanical ruggedness of them)

I'm wondering where you read  "to avoid short runs of high-eff coax"......
That is nonsense!
The only possible problem can arise from the stiffness and larger bend radius necessary when using a RG-8 size cable versus a smaller RG-58 or RG-8x size cable.
If you've got a small space for your shack and want things to be kind of "squished", then the larger cables can be a bit difficult to work with, but other than worries.

The long answer is:
The term "RG-8" is really only used nowadays to denote the "size" and or "type" of coax, since RG-8 is really an old obsolete name.......
SO, what you need to be aware of is:
1) Is that some OLD RG-8 coax, that should be in the trash can Huh?
2) Is that some new "generic" RG-8 coax??? (that should also be in the trash can...)

[remember even good quality coax will have some serious degardation over the years ~~~~~ and the best you could expect out of a "generic" RG-8 coax is 10 years.......less if it's been subject to a lot of sunlight, etc.]

3) Is it some new "RG-8 size" coax, that is just fine.
(such as Davis RF "Bury-Flex",  Belden 8337 or 9251, or even a RG-213, etc.Huh?)

4) Is it a "low-loss" RG-8 type cable?Huh
(foam, semi-solid/air-spaced, etc. dialectric???)

You should determine exactly what you bought, before you spend the time making jumpers out of it!

For 12 cents per foot, my guess is that is old coax that you might not want to use.....
How flexible is it?Huh  Does the jacket look worn at all???   Does it smell?Huh
Check the "shinnyness" of the copper braid and center conductor....if it's dull, it's OLD, and I'd not use it.
Also check the dialectric, if it's a "solid PE" it should look translucent....kind of like clear silicone.
If it's "milky", toss it in the trash!

If you did get a "new" 50' piece of RG-8, and you're satisfied that it's a good enough quality to use (i.e. NOT a "generic" RG-8) then the only problem I forsee is, connector application.
If, you've never soldered a PL-259 onto a RG-8 size cable, please ask for some help from a local experienced ham, or buy some cables with "professionaly installed" connectors (Radio Works or The Wireman).
At the very least, follow the directions that you'll find in any edition of the ARRL Handbook.  (you do have a copy of "The Handbook", don't you?Huh)
Use a good hot iron, with plenty of "mass" that stays hot, (or at least a 250watt gun)  and "silver-plated" connectors will allow the solder to flow quite nicely, but are not necessary.....
Be especially careful if your coax has a "foam" dialectric, since it will melt a lot quicker than you think......

I hope I helped some.
Good luck.

John,   KA4WJA


Posts: 3585

« Reply #3 on: September 21, 2002, 07:34:53 PM »

Hi Mike: No, you won't have problems with RG8 jumpers. Don't tell anyone but I cheat and use some half inch hardline jumpers I picked up for a buck each a few years ago. I had to pick up some right angle M-FM connectors to make them work but they are as high efficency as you can get and they work great.

But the next time you go to a hamfest keep a few bucks back and shop the guys with books. You never know what information you will need, badly, when there is no one around to ask. A good library is absolutely invaluable. A few bucks each for old radio engineering books, transmitting tube manuals, Radio Amateur Handbooks, and the like will eventually pay off big time.

73  Pete Allen  AC5E

Posts: 523


« Reply #4 on: September 21, 2002, 07:43:32 PM »

For the short cable's you might want to try RG8X. It is aboy as thin as RG/58. Under 30 MHZ you can even use the thinner lighter rg8/x for long runs.

   RG8/x has about the same electrrical contants as its full size big brother RG-8.

   It is just easier to work with in general for HF work.

   However for VHF/UHF either use full size RG8.
If a thin cable is still needed, take a look at RG-213.

     Once used RG/8 as an antennuator at 2GHZ. A 30 foot run at 2ghz WITH N  Connectors, gave me 10DB of antennuator.

     I like RG/8X in general. Radio shack even has solderless connectors for it. Not the best way, but an easy way to get from the rig to the antenna.

Posts: 10248


« Reply #5 on: September 21, 2002, 09:32:18 PM »

Oh boy, I'm going to get in trouble for this one....

Some folks try their best to minimize losses here there and everywhere. Think about this for a moment.

Everytime you use a PL259/SO239 combination, you lose from .1 to .2 db depending upon the type used. A typical shack has 8 or more in series.

Switching between coaxes of different velocity factors adds to this loss. So, instead of .1 or .2 db, you may have .2 or .3 db.

Barrel connectors are typically an additional .1 db.

It shouldn't take a rocket scientist to see a loss of 1, 2, or more db in the typical shack before the signal even gets to the antenna output terminal.

Then the question remains? Why worry about .05 db compared to .04 db or about the difference between RG8 and RG8X per foot at 30 Mhz?

Alan, KØBG


Posts: 29

« Reply #6 on: September 22, 2002, 03:59:01 PM »

I feel a little outgunned here, on account of I'm new and all. Hopefully I wont get chewed up too bad for stating the obvious. I'm a digital type, and this RF theory is fascinating but a tad confusing.

Anyhow, I went to the ham swap not to obtain top level stuff, but simply to get some materials to experiment with. By the way, coax is RG8/U Tandy. Its new in its box. Nice and shiny...

For the last few months I've been trying to come up with a reasonable antenna scheme. And, for K0BG, I think I read somewhere that most folks would be better off with a dipole, so I bought that too.

Anyway I strung up the dipole as follows:


Its a SpiRo trap dipole and I fed it with RG8U including a 8 turn loop balun at the feedpoint.

So far, all I hear is noise and the occasional hot signal. No contacts yet.

Am I doing anything dumb here?

73 Mike

Posts: 939

« Reply #7 on: September 22, 2002, 05:51:16 PM »

Are you doing anything "dumb"Huh
I doubt it!!

But, you may have a little problem to fix.
If you're using your 706 w/ your new trap dipole, you should definately be hearing more than "the occasional hot signal"!

Before we get to the specifics, my advice is to join your local ham radio club and get to know some of the local guys.
You will find just listening to a couple of hams with more experience, will give you a LOT of info that is very useful!  
{Also, I hope you have a copy of "The ARRL Handbook", if not, Please spend the $30 and get one!}
The arrl website is also a VERY useful resource, especially in this you could find a local/regional on-air net (say on 40m at noon local time?Huh) that would be a good way to test your least you'll be able to be sure that you'll hear someone, assuming all is okay with you antenna, etc.....

Okay, no onto the specifics....
How did you attach the connectors to the Tandy RG-8u coax?Huh  (My guess is, you may have a "short" in one or both connectors that is causing your lack of reception...)
Have you tried transmitting?Huh  
And testing your VSWR?Huh?
What bands and time of day have you tried listening on???  
As I'm writing this, at 5:50pm EDT Sunday 9-22-2002, 10, 15, and 20 meters are all open (very OPEN) and I can hear the SW Broadcasts on 40m, from Europe, coming in fairly well......Although, I'm in Central Florida, I assume that you should be hearing the same, except for 40m.......
If you're not, something is amiss.....
(I still suspect your PL-259's...)

Good luck and let us know more...
John,   KA4WJA

Posts: 2198

« Reply #8 on: September 22, 2002, 06:43:08 PM »

Hi Mike,
    I'd say there is something drastically wrong with your antenna. (Not necessarily your fault, though.)   The coax should be OK, (but not the best) at least for HF work.  (I'm being generous here.)
    Some considerations:

1.  What bands are you listening on?  (Remember, 10 and 15 Meters have not had good propagation this summer.)
2.  Can you at least borrow an antenna analyzer from another ham to check the SWR?
3.  Check for continuity (actually, if it's a "dipole" or inverted "V" what you want is LACK of continuity) between the center conductor and shield of the cable.
4.  This is a little harder, but check that there IS continuity between the center conductor of the PL-259 that goes to the radio and one side of the "dipole," and that there IS continuity between the shell of the PL-259 and the other side of the "dipole."  A broken connection between either could cause your problem.
5.  I have an antenna system much more modest than yours, (smaller, lower, and in a residential density neighborhood) yet I do get out (and hear a WHOLE lot better than I can transmit...).  20 Meters almost always has activity, even through this summer.  How is reception on 20M?  Also, how is reception on WWV at the various frequencies?
6.  I'm not familiar with the actual antenna you describe, but you mentioned it's a trap dipole.  Is it new, or used?  Either way, a trap or traps could be bad, causing the problem.
Good luck, and post again when you try some of the tips posted here, or find a solution.  You can always e-mail me for more details if you wish at :
Larry WA9SVD

Posts: 939

« Reply #9 on: September 24, 2002, 10:44:01 PM »

Not sure when you'll have the time to perform these tests and reply with what you find....
But, please do so....

We'll get it all working fine in no time....
Okay, I'm an optimist....Hi, hi.

John,  KA4WJA


Posts: 8

« Reply #10 on: September 25, 2002, 12:12:16 PM »

Hello Mike,
  I guess you made some good purchases at the River City Swap on Saturday.  I was there too and got a few bits and pieces at a good price.  No one has mentioned anything about the Ladder Line you bought.  In fact, it's probably the best thing to use to feed your dipole.  The reason the ladder line is good is that it is a balanced line feeding a balanced antenna and it will have low loss even if you don't have a perfectly matched antenna.  The published specs for loss in coax are for when you are feeding a resonant antenna.  If the antenna is not resonant, the losses can be terrible, even with the best coax.
   If you are going to use that trap dipole, the only additional piece of hardware you'll need between the ladder line and your radio is a balun.  You can either buy one or make one.  The Radio Place may still have some left, but they're closing shop next month.  HSC in Citrus Heights has them as well as parts to build one and they also have kits to build them (perhaps the best compromise between learning about baluns and ensuring what you make really works).  You could also use one that is built into a tuner.  Finally, even if the trap dipole says its made for the bands advertised, make sure you measure the real SWR with an SWR meter before you TX  on it.  There are so many local interactions between the antenna and nearby objects that the SWR curve is almost never exactly as advertised and you may want to have an antenna tuner handy to keep your rig's finals happy.

Good luck,

Posts: 29

« Reply #11 on: September 28, 2002, 05:02:47 PM »

Hello again, sorry for the lag time. I have since logged a few contacts on this modest arrangement. I should say that I've been soldering practically since I could crawl, and acquired knowledge of Ohms law and a couple of dozen DVMs since then. The antenna is actually working. Obviously, I checked for shorts before even attempting to TX. My main problem is lack of understanding of propagation, operating modes, and HF protocol. (I got scolded for CQ on an active freq... In my defense, I could only hear one side...).

I also took your advice and purchased four different books from the local shop. They are complex, but fun, and, a great learning tool. Espescially in regards to anttenae.


Posts: 2198

« Reply #12 on: September 30, 2002, 09:13:05 PM »

I don't think the ladder line is a good idea to use for the antenna in question!  It's a trap dipole, so the impedence is going to be in the vicinity of 50-75 Ohms, depending upon height.  Ladder line, while low loss, and balanced, has a characteristic impedence of either 300, 450, or 600 Ohms.  That would present a poor match to a dipole.  A balun (1:1) might be in order, but the ladder line isn't the best choice for a dipole system.  There ARE designs that use ladder line in a "dipole" arrangement, but they require specific lengths of Wire AND Ladderline feedline (such as the G5RV.)
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